Re: [PATCH v5 2/7] x86/msr: add wrmsrq_on_cpus helper

["Kalra, Ashish" <ashish.kalra@amd.com>]

Hello Boris and Dave,

On 5/27/2026 7:43 PM, Borislav Petkov wrote:
> On Wed, May 27, 2026 at 02:38:05PM -0700, Dave Hansen wrote:
>> This one is my doing.
> 
> I know.
> 
> But hey, maybe we should not disagree on the public ML because the submitter
> might disappear like the last one. :-P
> 
>> wrmsr_on_cpus() is kinda a mess. I think it only has a single user. It's
>> also not very flexible because it needs a 'struct msr __percpu *msrs'
>> argument where each MSR has a value in memory.
> 
> Right, we did that a looong time ago.
> 
> The only reason I'd have for per-CPU MSR structs is reading different MSR
> values on different cores, modifying only the bits you need and then *keeping*
> the remaining values as they were. And that interface allows you to do that
> while this new thing won't.
> 
> And I'm going to venture a guess here that adding a simpler interface which
> simply forces a new value ontop of a whole MSR could cause a lot of subtle
> bugs when people don't pay attention to keep the old values.
> 
>> The use case for RMPOPT is that all CPUs get the same value. It'd be a
>> little awkward to go create a percpu data structure to duplcate the same
>> value to call wrmsr_on_cpus(). The RMPOPT case is also arguably
>> performance sensitive since it's done during boot. It should do the IPIs
>> in parallel.
> 
> Oh sure, my meaning was to create something that serves both purposes.
> 
>> toggle_ecc_err_reporting(), on the other hand, is done at module init
>> time. It's not really performance sensitive. It's probably pretty easy
>> to zap wrmsr_on_cpus() and just have toggle_ecc_err_reporting() do
>> something slightly less efficient.
> 
> Sure. That's fine.
> 
>> Yeah, the
>>
>> 	wrmsr_on_cpus()
>> 	wrmsrq_on_cpus()
>>
>> naming pain is real. There's little chance of bugs coming from it
>> because the function signatures are *SO* different. But, it certainly
>> could confuse humans for a minute.
> 
> Yap.
> 
>> But the real solution to this is axing wrmsr_on_cpus(). 
> 
> Yap, for example. Basically reingeneering the whole
> write-MSRs-on-multiple-CPUs functionality is what I meant.
> 
>> Which I think we could do after killing its one user which the attached
>> (completely untested) patch does. The only downside of the patch is that it
>> does RDMSR via IPIs one CPU at a time. But, looking at the code, I'm not
>> sure anyone would care. If anyone did, I _think_ all those MSRs have the
>> same value and the code could be simplified further. But that would take
>> more than 3 minutes.
>>
>> It's also possible that my grepping was bad or I'm completely
>> misunderstanding amd64_edac.c. Cluebat welcome if I'm being dense.
> 
> Looks ok to me, we can surely do that. I even hw to test it. I think...
> 
>> BTW, I also don't feel the need to make Ashish go do any of this edac
>> cleanup. I think it can just be done in parallel. But I wouldn't stop
>> him if he volunteered.
> 
> Why not?
> 
> It has always been the case: cleanups and bug fixes first, new features ontop.
> 
> So yeah, modulo figuring out how to redefine the *msr_on_cpus() interface,
> I think this all makes sense.

snp_setup_rmpopt() runs once during init and rmpopt_cleanup() runs once during shutdown. The batch IPI optimization
is irrelevant here. This RMPOPT_BASE MSR setup/programming is not in a performance critical path.

A simple loop would be perfectly fine and avoids the need for the wrmsrq_on_cpus() helper entirely:

  for_each_cpu(cpu, &rmpopt_cpumask)
      wrmsrq_on_cpu(cpu, MSR_AMD64_RMPOPT_BASE, rmpopt_base);

Calling wrmsrq_on_cpus() here for programming RMPOPT_BASE MSR:

-       wrmsrq_on_cpus(&rmpopt_cpumask, MSR_AMD64_RMPOPT_BASE, rmpopt_base);
+       for_each_cpu(cpu, &rmpopt_cpumask)
+               wrmsrq_on_cpu(cpu, MSR_AMD64_RMPOPT_BASE, rmpopt_base);

So i will drop this helper patch.

Thanks,
Ashish

> 
> Thx.
>